The present invention generally relates to a pick-up arrangement and more particularly, to a pick-up device suitable for use in an optical disc apparatus such as a so-called compact disc player, video disc player or the like.
Commonly, an optical disc apparatus is arranged so as to effect recording by forming fine holes or grooves in a spiral or concentric pattern on the surface of a disc-like recording medium or by subjecting the surface of the recording medium to magnetic variations, and also to effect reproduction by projecting a collected spot light onto the recorded portion of the recording medium so as to derive the reflected light or transmitted light as reproduction information.
In FIG. 1, there is shown a general construction of a conventional optical information reading pick-up device as incorporated in an optical disc apparatus.
In the known pick-up device as shown in FIG. 1, the optical system includes a collimate lens Lm and a beam splitter Sp provided in a barrel Br so as to collect and project laser light emitted from a semi-conductor laser W onto a recording medium M through said collimate lens Lm, said beam splitter Sp and an objective lens Lo. After being subjected to modulation at the information recorded portion on the recording medium M, the light thus projected is divided by the beam splitter Sp through the objective lens Lo, and one portion of the divided light is incident upon a photo-detector Pd via a detecting system convex lens Ld and a cylindrical lens Lc.
In the conventional optical information reading pick-up device having the optical system as described above, in order to correctly record the information on the recording medium or so as to accurately reproduce the recorded information therefrom, it is necessary to control in such a manner that the collected light spot correctly traces the information portion (referred to as tracking) as recorded in the sprial or concentric configuration (i.e. information recorded track), with a simultaneous control for focusing of the light spot.
Moreover, for the actual application, since the above pick-up device is subjected to influences due to eccentricity or fluttering of the disc, and external vibrations, etc., it is required to provide a mechanism to derive such deviations as error signals for automatic correction. For this purpose, it has been a normal practice to effect the focus control and the tracking control through employment of an electromagnet driving unit including a magnetic circuit and a coil means. For effecting control up to the high frequency region during the above tracking driving, it is desirable that the driving portion is compact in size and light in weight.
From the above point of view, there has also been conventionally proposed an arrangement in which only the objective lens Lo is separated from the barrel for driving, but in such a construction, deviation in axes is produced between an optical axis of the pick-up and that of the objective lens, thus resulting in deterioration in the quality of the pick-up control signal, for which some countermeasures are required. Furthermore, in order to avoid the deterioration in the recording signal and reproducing signal with respect to the disc, a driving unit having a sufficiently high accuracy is required so as to prevent inclination of the lens optical axis from taking place during driving of the objective lens. Meanwhile, in the pick-up device as shown in FIG. 1, in which the whole pick-up assembly is driven, cross-talk between the tracking control and the focus control tends to take place since the barrel Br is not formed with axial symmetry. More specifically, on the assumption that the pick-up is driven in a direction at right angles with the recorded surface of an information carrier or recording medium having a disk-like shape, a phenomenon where the optical axis deviates from the direction normal to the recorded surface, is difficult to avoid when a pick-up device is used which does not have axial symmetry.
Additionally, in the prior art pick-up device as shown in FIG. 1, the astigmatism producing optical system including the convex lens Ld and the cylindrical lens Lc is required to obtain the focusing error signal for effecting the focus control. However, such an optical system becomes an obstruction in realizing the reduction of size, weight and cost for the pick-up device.